Touch screens are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers.
FIG. 1 shows a multilayer 10 for a typical prior art resistive-type touch screen including a transparent substrate 12, having a first conductive layer 14. A flexible transparent cover sheet 16 includes a second conductive layer 18 that is physically separated from the first conductive layer 14 by spacer elements 20. A voltage is developed across the conductive layers. The conductive layers 14 and 18 have a resistance selected to optimize power usage and position sensing accuracy. Deformation of the flexible cover sheet 16 by an external object such as a finger or stylus causes the second conductive layer 18 to make electrical contact with first conductive layer 14, thereby transferring a voltage between the conductive layers. The magnitude of this voltage is measured through connectors (not shown) connected to metal conductive patterns (not shown) formed on the edges of conductive layers 18 and 14 to locate the position of the deforming object.
It is known from U.S. Pat. No. 5,354,613 to employ antistatic coatings containing conductive polythiophene polymeric materials on resin or paper and that such materials may contain matting agents for anti-blocking purposes. It is also known from EP 1,079,397 to use conductive polythiophene polymeric materials to provide an electroconductive pattern on a support and that such materials may contain spacers, UV filters or IR absorbers. It is further known from U.S. Pat. Nos. 5,674,654 and 5,665,498 to use conductive polypyrrole polymeric materials that may contain matting agents as combination antistat/protective layers for imaging elements.
The conventional construction of a resistive touch screen involves the sequential placement of materials upon the substrate. The substrate 12 and cover sheet 16 are first cleaned, then uniform conductive layers are applied to the substrate and cover sheet. It is known to use a coatable intrinsically conductive polymer such as polythiophene or polyaniline to provide the flexible conductive layers. See for example WO 00/39835, which shows a light transmissive substrate having a light transmissive conductive polymer coating, and U.S. Pat. No. 5,738,934 which shows a cover sheet having a conductive polymer coating. The spacer elements 20 are then applied and, finally, the flexible cover sheet 16 is attached.
The deposition of spacer elements between the substrate and flexible cover sheet can be accomplished in several ways, for example by spraying through a mask or pneumatically sputtering small diameter transparent glass or polymer particles, as described in U.S. Pat. No. 5,062,198. The transparent glass or polymer particles are typically 45 microns in diameter or less and mixed with a transparent polymer adhesive in a volatile solvent before application. This process is relatively complex and expensive and the use of an additional material such as an adhesive can be expected to diminish image quality.
Some displays, such as those composed of organic light emitting diodes (OLEDs) are sensitive to ultra-violet light. Other displays may require selective filtration of incident or emitted light in order to provide improved imaging quality.
In general, any light with a wavelength less than 400 nm is capable of degrading the organic light emitting diode materials, reducing the lifetime of the display. To reduce the effects of this problem, oxygen may be excluded from OLED devices to the extent possible. UV filters may also reduce the radiation incident on the organic materials in OLED displays and hence the incidence of photo-oxidation. However, the addition of an additional UV filtering layer is undesirable because the addition of another layer tends to degrade image quality and increase costs and manufacturing steps.
Materials used to construct both displays and touch screens may filter light passing through them so that a perceptible color is imparted to the light. For example, when a flat-panel display is viewed through a touch screen, the colors of the flat-panel display may be altered so that the colors are less accurate or the color gamut is reduced. It is generally preferable, if the materials are not perfectly transparent, that they combine to provide a neutral density. Alternatively, under known, relatively invariant light conditions, it may be preferable to induce a particular overall color to light emitted by a display with the use of suitable filters.
It is a problem to be solved to provide an improved touch screen and method of making the same that reduces the difficulty of placing spacer elements while maintaining or improving image quality.